Current handling of the random access response window imposes unnecessary restrictions on the network and user equipment nodes (UEs).
Long Term Evolution (LTE) uses Orthogonal Frequency Division Multiplexing (OFDM) in a downlink and Discrete Fourier Transform (DFT) spread OFDM in the uplink. The basic LTE downlink physical resource can thus be seen as a time-frequency grid 101 as illustrated in FIG. 1, where each resource element 103 corresponds to one OFDM subcarrier during one OFDM symbol interval.
Referring to FIG. 2, in the time domain, LTE downlink transmissions are organized into radio frames 201 of 10 ms, each radio frame consisting of ten equally-sized subframes 2030 to 2039, each of length Tsubframe=1 ms.
Furthermore, the resource allocation in LTE is typically described in terms of resource blocks (RB), where a resource block corresponds to one slot (0.5 ms) in the time domain and 12 contiguous subcarriers in the frequency domain. A pair of two adjacent resource blocks in the time direction (1.0 ms) is known as a resource block pair. Resource blocks are numbered in the frequency domain, starting with “0” from one end of the system bandwidth.
The notion of virtual resource blocks (VRB) and physical resource blocks (PRB) has been introduced in LTE. The actual resource, allocation to a UE is made in terms of VRB pairs. There are two types of resource allocations, localized and distributed. In the localized resource allocation, a VRB pair is directly mapped to a PRB pair, hence two consecutive and localized VRB are also placed as consecutive PRBs in the frequency domain. On the other hand, the distributed VRBs are not mapped to consecutive PRBs in the frequency domain; thereby providing frequency diversity for data channel transmitted using these distributed VRBs.
Downlink transmissions are dynamically scheduled. That is, in each subframe the base station transmits control information relating to which terminals data is transmitted and upon which resource blocks the data is transmitted, in the current downlink subframe. This control signaling is typically transmitted in the first 1, 2, 3 or 4 OFDM symbols in each subframe and the number n=1, 2, 3 or 4 is known as the Control Format Indicator (CFI). The downlink subframe also contains common reference symbols (CRS), which are known to the receiver and used for coherent demodulation of e.g. the control information. A downlink system with CFI=3 OFDM symbols as control is illustrated in FIG. 3.
The release 10 of the LTE specifications (LTE Rel-10) have been standardized, supporting Component Carrier (CC) bandwidths up to 20 MHz (which is the maximal LTE Rel-8 carrier bandwidth). An LTE Rel-10 operation wider than 20 MHz is possible and appears as a number of LTE CCs to an LTE Rel-10 terminal. One way to obtain bandwidths wider than 20 MHz is by means of Carrier Aggregation (CA). CA implies that an LTE Rel-10 terminal can receive multiple CC, where the CC have, or at least the possibility to have, the same structure as a Rel-8 carrier. CA is illustrated in FIG. 4, whereby five 20 MHz CCs 4011 to 4015 are shown as being aggregated to provide a bandwidth of 100 MHz.
The Rel-10 standard of LTE support up to 5 aggregated CCs where each CC is limited in the RF specifications to have a one of six bandwidths namely 6, 15, 25, 50, 75 or 100 RB (corresponding to 1.4, 3 5 10 15 and 20 MHz respectively).
The number of aggregated CCs as well as the bandwidth of the individual CCs may be different for uplink and downlink. A symmetric configuration refers to the case where the number of CCs in downlink (DL) and uplink (UL) is the same whereas an asymmetric configuration refers to the case that the number of CCs is different in downlink and uplink. The number of CCs configured in the network may be different from the number of CCs seen by a terminal: A terminal may, for example, support more downlink CCs than uplink CCs, even though the network offers the same number of uplink and downlink CCs.
CCs are also referred to as cells or serving cells. More specifically, in an LTE network the cells aggregated by a terminal are denoted primary Serving Cell (PCell) and secondary Serving Cells (SCells). The term serving cell comprises both PCell and SCell. All UEs have one PCell and which cell is a UEs PCell is terminal specific and is considered “more important”, i.e. vital control signaling and other important signaling is typically handled via the PCell. Uplink control signaling is sent on a UEs PCell. The component carrier configured as the PCell is the primary CC whereas all other component carriers are secondary serving cells.
During initial access a LTE Rel-10 terminal behaves similar to a LTE Rel-8 terminal. However, upon successful connection to the network a Rel-10 terminal may—depending on its own capabilities and the network—be configured with additional serving cells in the uplink and downlink. Configuration is based on radio resource control (RRC). Due to the heavy signaling and rather slow speed of RRC signaling it is envisioned that a terminal may be configured with multiple serving cells even though not all of them are currently used.
Random Access
In LTE, as in any communication system, a mobile terminal may need to contact the network (via the eNodeB) without having a dedicated resource in the Uplink (from UE to base station). To handle this, a random access procedure is available where a UE that does not have a dedicated uplink resource may transmit a signal to the base station. The first message (MSG1 or preamble) of this procedure is typically transmitted on a special uplink resource 501 reserved for random access, a physical random access channel (PRACH), with other uplink resources 503 being used for data transmission. This channel can for instance be limited in time and/or frequency (as in LTE), as shown in FIG. 5.
The resources available for PRACH transmission is provided to the terminals as part of the broadcasted system information (or as part of dedicated RRC signaling in case of handover, for example).
In LTE, the random access procedure can be used for a number of different reasons. Among these reasons are:                Initial access (for UEs in the LTE_IDLE or LTE_DETACHED states)        Incoming handover        Resynchronization of the UL        Scheduling request (for a UE that is not allocated any other resource for contacting the base station)        Positioning        
A contention-based random access (CBRA) procedure used in LTE is illustrated in FIG. 6. The UE starts the random access procedure by randomly selecting one of the preambles available for contention-based random access. The UE then transmits the selected Random Access Preamble message 601 on the physical random access channel (PRACH) to a network node such as an eNode B in the network.
The network acknowledges any preamble it detects by transmitting a Random Access Response message 603 (MSG2) including an initial grant to be used on the uplink shared channel, a temporary Cell-Radio Network Temporary Identifier (C-RNTI), and a time alignment (TA) update based on the timing offset of the preamble measured by the eNodeB on the PRACH. The Random Access Response message 603 (MSG2) is transmitted in the downlink to the UE and its corresponding Physical Downlink Control Channel (PDCCH) message's Cyclic Redundancy Check (CRC) is scrambled with the Random Access-Radio Network Temporary Identifier (RA-RNTI).
When receiving the Random Access Response message 603 (MSG2) the UE uses the grant to transmit a Scheduled Transmission message 605 (MSG3) that in part is used to trigger the establishment of radio resource control and in part to uniquely identify the UE on the common channels of the cell. The timing advance command provided in the Random Access Response message 603 is applied in the UL transmission in the Scheduled Transmission message 605 (MSG3). The eNodeB can change the resources blocks that are assigned for a Scheduled Transmission message 605 (MSG3) by sending an uplink grant, the CRC of which is scrambled with the Temporary Cell-Radio Network Temporary Identifier (TC-RNTI).
The Contention Resolution message 607 (MSG4) then has its PDCCH CRC scrambled with the C-RNTI if the UE previously has a C-RNTI assigned. If the UE does not have a C-RNTI previously assigned, it has its PDCCH CRC scrambled with the TC-RNTI.
The procedure ends with the network solving any preamble contention that may have occurred for the case that multiple UEs transmitted the same preamble at the same time. This can occur since each UE randomly selects when to transmit and which preamble to use. If multiple UEs select the same preamble for the transmission on RACH, there will be contention between these UEs that needs to be resolved through the Contention Resolution message 607 (MSG4). The case when contention occurs is illustrated in FIG. 7, where two UEs 7011 and 7012 transmit the same preamble, p5, at the same time. A third UE 7013 also transmits at the same RACH, but since it transmits with a different preamble, p1, there is no contention between this UE 7013 and the other two UEs 7011 and 7012.
It is noted that a UE can also perform non-contention based random access. A non-contention based random access or contention free random access (CFRA) can, for example, be initiated by the eNodeB to get the UE to achieve synchronisation in the uplink. The eNodeB initiates a contention free random access either by sending a PDCCH order or indicating it in an RRC message. The later of the two is used in case of handover.
The eNodeB can also order the UE through a PDCCH message to perform a contention based random access; the procedure for this being illustrated in FIG. 8. The eNodeB transmits a Random Access Order message 801 to a UE, and a UE will transmit a Random Access Preamble message 803 to the eNodeB. Similar to the contention based random access described in FIG. 6, a Random Access Response message 805 (MSG2) is transmitted in the downlink to the UE and its corresponding PDCCH message CRC is scrambled with the RA-RNTI. The UE considers the contention resolution successfully completed after it has received the Random Access Response message 805 (MSG2) successfully.
For the contention free random access, in a similar manner to the contention based random access, the Random Access Response message 805 (MSG2) contains a timing alignment value. This enables the eNodeB to set the initial/updated timing according to the UEs transmitted preamble.
In LTE in Rel-10 the random access procedure is limited to the primary cell only. This implies that the UE can only send a preamble on the primary cell. Further a Random Access Response message 603 (MSG2) and a Scheduled Transmission message 605 (MSG3) is only received and transmitted on the primary cell. A Contention Resolution message 607 (MSG4) can, however, in Rel-10 be transmitted on any downlink cell.
In LTE Rel-11 random access procedures will be supported also on secondary cells, at least for the UEs supporting Rel-11 carrier aggregation. So far only network initiated random access on SCells is assumed.
Random Access Response Window
After a UE has sent a preamble it listens for a random access response from the network for a certain time, this time period given by the value of a random access response window. After a time equal to the random access response window has passed the UE considers the preamble transmission to be unsuccessful and resends the preamble. Each time the UE resends the preamble the UE will increase the output power used to transmit the preamble to increase the chance of a successful preamble transmission. The UE will transmit a maximum number of preambles given by a value “preamble transmission maximum”.
In LTE Rel-10 with the introduction of carrier aggregation the UE is able to aggregate multiple carriers, i.e. to be configured with a PCell and SCells. For LTE Rel-11 a UE is able to perform random access procedures on SCells. In contrary to the PCell, however, there is no way of signaling the parameters necessary for performing a random access procedure on SCells. In particular, the RA response window value is currently discussed in 3GPP whether it shall be signaled for SCells or not. If it is signaled for SCells the UE would use the RA response window value for the SCell where the RA procedure is performed while if the RA response window is not signaled for SCells the UE would apply the RA response window value for the PCell when performing RA on SCells. If the UE applies a cell specific RA response window the network implementation might become more complex than the case if a UE always applies the RA response window of the PCell also for RA procedures performed on SCells. However, having cell specific RA response windows will increase flexibility. One network vendor, vendor A, might prefer to avoid this extra complexity and hence prefer to not signal RA response window values for SCells. Another network vendor, vendor B, might judge that the extra complexity is justified to obtain the added flexibility it would mean to have RA response window values for SCells. If the LTE specification is specified so that different cells are configured with different SCell RA response windows, vendor A is forced to specify RA response windows for different SCells. If the LTE specification is specified so that the UE applies the PCells RA response window also for SCell RA procedures, vendor B will not have the flexibility it wants. A disadvantage of this is that both vendor A and vendor B's needs cannot be met with current solutions.
As described above two types of RA procedures are defined; contention based random access (CBRA) and non-contention based random access or contention free random access (CFRA). These two types of RA procedures are used for different purposes and in different situations. For example CBRA is used by UEs when performing initial access to the network while CFRA is used when a UE is achieving uplink time alignment on SCells in a secondary TA group. With current solutions the UE applies the same RA response window for these two types of RA which puts unnecessary restrictions on the network and UE.